MEDICAL Q&A: Your sense of smell is highly personal — and variable

Tuesday

Nov 14, 2017 at 2:01 AM

Q: Does a person's sense of smell vary according to the time of day?

A: It can. When people tell you, “wake up and smell the roses,” they might be giving you bad advice. Your sense of smell may fluctuate in sensitivity over the course of 24 hours, in tune with our circadian clocks, with your nose best able to do its job during the hours before you go to sleep, according to a new study.

The work, reported in the journal Chemical Senses, is part of a larger push to explore whether adolescents’ senses of taste and smell influence obesity. Rachel Herz, a sensory researcher at Brown University, and her colleagues designed this study to see if there might be times of day when the sense of smell was more powerful — perhaps making food smell particularly inviting.

For the experiment, 37 adolescents ranging in age from 12 to 15 came into a lab for a very long sleepover party. For nine days, they followed a strict schedule to allow researchers to focus on the circadian clock, which helps control wake and sleep, but also influences other processes in the body, including metabolism.

While more research is needed to test whether the results fully apply to adults, Herz says that as you grow up, the makeup of the smell receptors inside your nose doesn’t seem to change, although there is evidence your body clock may.

The team kept track of where the teenagers were in their circadian cycle by measuring their saliva’s levels of melatonin, a hormone that rises and falls regularly over the course of the day. Every few hours, the children took a scent test, sniffing different concentrations of a chemical that smells like roses. The researchers recorded the lowest concentration they could detect at each time point.

When the results were tallied up, the researchers saw a range of responses.

“Nobody has the same nose,” Herz said. Some adolescents had only very mild changes in sensitivity, while sensitivity altered dramatically in others.

Averaged together, however, the results showed that overall the circadian clock does affect smell, and that the times when the children’s noses were most sensitive tended to correspond to the evening, with an average peak of 9 p.m.

“The results make sense — the circadian clock affects virtually every organ system in the body,” writes Leslie Vosshall, a researcher at Rockefeller University who studies smell and was not involved in the study, in an email.

Smell was at its lowest ebb, intriguingly, from about 2 a.m. to 10 a.m.

It is already known that when we are asleep, a strong smell won’t disturb us the way a loud noise or a bright light will. Perhaps the biological machinery behind smell shuts itself down for the night, at least in some people. But Herz speculates that having stronger olfactory abilities as dusk fell might have helped our ancestors survive.

Still, the experiment was designed to test the effect of the circadian clock, and that is not the only factor involved in smell sensitivity. Researchers have already found that another big player is how long someone has been awake and what variety of smells they have been exposed to. It’s likely that all of these have a role in determining when, in real life outside the lab, our sense of smell works best.

— Veronique Greenwood, The New York Times

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